This invention relates to a printing apparatus and a printing method and, more particularly, to a printing apparatus and method for printing a binary image.
In an example of processing according to the prior art, four-pass printing is carried out while executing multi-pass print control by a printer using a printhead having eight nozzles for printing image data, which has been obtained by binarizing input multi-valued image data using a dither matrix composed of 4xc3x974 dots, by discharging ink in accordance with the ink-jet method.
FIG. 8 is a diagram showing an example of a binarized image in which pseudo-halftone representation is achieved using a dither matrix of 4xc3x974 dots. This example is illustrative of a case where an original image of a uniform scene has been binarized with comparatively strong highlighted density and shows a halftone image of a repeating pattern of seven black pixels in a dither matrix of 4xc3x974 dots.
FIGS. 9A-9D are diagrams showing mask patterns when multi-pass control is carried out, particularly for four-pass printing.
In FIGS. 9A-9D, FIG. 9A illustrates a mask pattern used in a first pass, FIG. 9B a mask pattern used in a second pass, FIG. 9C a mask pattern used in a third pass and FIG. 9D a mask pattern used in a fourth pass.
Reference will now be had to FIGS. 10A-10G to describe the progress of processing in a case where the halftone image shown in FIG. 8 is printed by multi-pass (four-pass) printing by an eight-nozzle printhead using mask patterns of the kind shown in FIGS. 9A-9D.
FIGS. 10A-10G are diagrams showing the manner in which multi-pass printing proceeds. FIG. 10A shows the printing result obtained from the first pass, FIG. 10B the printing result obtained from the first and second passes, FIG. 10C the printing result obtained from the first through third passes, and FIG. 10D the printing result obtained from the first through fourth passes. It should be noted that the rectangular shape with the numerals 1 through 8 at the upper right of the drawings of FIGS. 10A through 10G represents a printhead and that each of the numerals indicates a nozzle number.
In the example of four-pass printing using the eight-nozzle printhead of the kind shown in FIGS. 10A through 10G, a line feed equivalent to two nozzles is performed per scan of the printhead and therefore the printing of the first two lines is completed by the four scans of the printhead that are shown in FIGS. 10A-10D. Similarly, two lines composed of the third and fourth lines are printed in their entirety by the four scans of the printhead that are shown in FIGS. 10B-10E; two lines composed of the fifth and sixth lines are printed in their entirety by the four scans of the printhead that are shown in FIGS. 10C-10F; and two lines composed of the seventh and eighth lines are printed in their entirety by the four scans of the printhead that are shown in FIGS. 10D-10G.
In the binary images illustrated in FIGS. 10A-10G, the portions in which numerical values are shown indicate the portions printed by the scan of the printhead at that particular time; the numerical value indicates which nozzle performed printing. The hatched portions indicate portions already printed by preceding scans of the printhead.
Thus, printing equivalent to a total of eight lines is completed by the scans of the printhead in FIGS. 10A through 10G.
FIG. 11 is a diagram illustrating which nozzles discharged the ink that formed the printed dots obtained by the above-described printing operation.
In accordance with FIG. 11, it will be understood that the printing of the first line is performed by the nozzles 1 and 7; that of the second line by the nozzle 8 alone; that of the third line by the nozzles 1 and 7; that of the fourth line by the nozzles 2 and 6; that of the fifth line by the nozzles 3 and 5; that of the sixth line by the nozzle 4 alone; that of the seventh line by the nozzles 3 and 5; and that of the eighth line by the nozzles 2 and 6.
Printing is performed over the entire print medium by subsequently repeating the eight-line printing procedure described above.
The goal of multi-pass printing will be summarized in simple terms.
With a plurality of nozzles provided on a print head, the direction of ink discharge from the nozzles and the amount of ink discharge vary from nozzle to nozzle. Consequently, even when a single horizontal line is printed by a single nozzle, the horizontal line will have a fainter color than other horizontal lines if this nozzle happens to discharge an amount of ink that is small in comparison with the amount of ink discharged by the other nozzles. Such printing results in the appearance of a faint horizontal stripe at the portion printed by the nozzle in the case of a print image obtained by discharging ink over the entire surface of a print medium or an image having a uniform density. The end result is a decline in image quality.
The goal of multi-pass printing is to avoid this situation by printing the same line using a plurality of nozzles, whereby the amount of ink discharged on the line is made uniform overall and the variation in the characteristic of a specific nozzle is rendered inconspicuous.
If such multi-pass printing is controlled in such a manner that the same line is printed by a large number of nozzles, the effects caused by a fluctuation in the amount of ink discharged from a specific nozzle will be dispersed so as not to concentrate on a specific line. The result is that the printed image can be maintained at high quality.
In the example of the prior art described above, however, it should be obvious from the printed results shown in FIG. 11 that the printing of specific lines (the second and sixth lines in FIG. 11) uses only specific nozzles (nozzles 8 and 4, respectively, in FIG. 11). This means that the original goal of multi-pass printing is not attained and that a decline in print image quality may result.
The reason for such a degradation of the image is that while the dither matrix has a 4xc3x974 dot configuration, the size of the mask patterns used in multi-pass printing is 8xc3x978 dots. More specifically, since the matrix of the mask patterns for multi-pass printing has a size that is a whole-number multiple of the size of the dither matrix in both the horizontal direction (the scanning direction of the printhead, also referred to as the xe2x80x9cmain-scan directionxe2x80x9d) and vertical direction (the direction in which the printing paper is conveyed, also referred to as the xe2x80x9csub-scan directionxe2x80x9d), the printed pattern that is generated periodically by the dither matrix coincides with the periodicity of the nozzles selected by the mask patterns for multi-pass printing and, as a consequence, a situation arises in which in the same line is always printed by the same nozzle.
Accordingly, it is an object of the present invention to provide a printing apparatus and method in which a high-quality image can be printed at all times even in a case where image data is binarized by a binarization method for representing pseudo half tone and the binarized image is printed by multi-pass printing.
According to one aspect of the present invention, the foregoing object is attained by providing a printing apparatus for printing an image on a print medium using a pattern used for representing half tone, comprising: a printhead having a plurality of printing elements; and print control means for scanning a different printing element in the plurality of printing elements of the printhead on the same area of the print medium plural times and performing complementary printing using the pattern used for representing half tone and a mask pattern by the plural times of scans of the printhead; wherein the relationship between a period of a matrix of the pattern used for representing half tone and that of a matrix of the mask pattern is relatively prime.
It is preferred that the print control means includes thinning out means for complementarily thinning out the pattern used for representing half tone by using the mask pattern.
It is also preferred that when a size of the matrix of the pattern used for representing half tone is N1xc3x97M1, and that of the matrix of the mask pattern is N2xc3x97M2, at least one of the relationship between N1 and N2, and the relationship between M1 and M2 is relatively prime.
Further, it is preferred that not only the relationship between N1 and N2 is relatively prime, but also the relationship between M1 and M2 is relatively prime.
Furthermore, it is preferred that N1 and N2 are sizes with respect to a scanning direction of said printhead, while M1 and M2 are sizes with respect to a direction in which the print medium is conveyed.
It is preferred that the above apparatus further comprises conversion means for inputting multi-valued image data and outputting a pattern representing half tone. In this case, the conversion means outputs the pattern representing half tone by binarizing the multi-valued image data by using a dither matrix or a density pattern.
It is preferred that the above apparatus further comprises memory means for storing the pattern used for representing half tone and the matrix of the mask pattern.
It is preferred that the printhead has a plurality of ink discharge nozzles for discharging ink as the plurality of printing elements. In such case it is desirable that the printhead discharges the ink by utilizing thermal energy.
According to another aspect of the present invention, the foregoing object is attained by providing a printing method for printing an image on a print medium using a pattern used for representing half tone, comprising the steps of: scanning a different printing element in a plurality of printing elements included in a printhead on the same area of the print medium plural times; and controlling to perform complementary printing using the pattern used for representing half tone and a mask pattern by the plural times of scans of the printhead, wherein the relationship between a period of a matrix of the pattern used for representing half tone and that of a matrix of the mask pattern is relatively prime.
In accordance with the present invention as described above, printing is performed in such a manner that the periodicity of the matrix of the pattern used for representing half tone and the periodicity of the matrix of mask patterns used in multi-pass printing will not coincide. Furthermore, printing is performed by assigning a large number of different printing elements to the same line by arranging it so that a printing operation by each printing element of the printhead is dispersed over the scanning direction of the printhead.
The invention is particularly advantageous since a specific ink discharge nozzle will not happen to focus on the printing of a specific line. Even if ink is discharged from this nozzle abnormally, the abnormality will not happen to concentrate on a specific line, thereby making it possible to prevent a decline in the quality of the printed image.
Thus, the original goal of multi-pass printing can be attained and high-quality image printing can be carried out.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.